The selectivity and conversion of the sulfuric acid-catalyzed Beckmann rearrangement of cyclohexanone oxime, dissolved in cyclooctane, to ¿-caprolactam are determined in a microreactor for conditions with a high concentration of ¿-caprolactam. The microreactor consists of a low-temperature mixing zone followed by a high-temperature reaction zone. The mixing is conducted in a split-and-recombine micromixer and a microchannel at 65¿°C, followed immediately by a second microchannel at 100–127¿°C to obtain complete conversion. Under these conditions a selectivity of 99¿% is achieved. The residence time of the reactants in the microreactor setup is about 10¿s. In literature, a selectivity of about 95¿% for the same reaction in a similar setup is reported, but at a uniform temperature of 120–130¿°C for mixing and reaction. So, suppressing the reaction during mixing is a major tool to enhance the selectivity to ¿-caprolactam.